There is no way for Studio One to directly open Reason song files, so in Reason you would need to export all the audio tracks as wav files and export the midi. If you use any rack instruments/effects it would be a good idea to either save their current settings as presets or combine your instruments/effect chains in combinators and save those as combinator patches. In Studio One you would then create an empty song, import all the wav files and the midi, and assign instances of the Reason Rack Plugin to the midi tracks, using your newly saved presets.
